This proposal discusses the production and implementation of peptide/recombinant antigens for the development of a serological assay for the diagnosis of human babesiosis, a tick-borne disease. Increased surveillance of tick borne infections has shown that many individuals infected with Lyme disease have co-infections with Babesia microti (agent of human babesiosis) and Ehrlichia both of which are potentially fatal in immunocompromised individuals any may impact blood transfusions. Since the treatment of B. microti infections (malaria-like intraerythrocytic parasite) is vastly different to Lyme disease there is a need for an accurate serology test to differentiate the two infections. B. microti specific peptides/recombinants will be used alone or in combination to develop an assay to supplement the currently available Lyme disease test. Development of specific antigens for B. microti for use in diagnosis improves our ability to detect tick-borne diseases and facilitates administration of the appropriate treatment. Advances have already been achieved at Corixa in identifying immunodominant recombinant antigens. This in combination with a collaboration with key investigator in the area of tick-borne diseases and access to an extensive serum bank accumulated through work on other infectious disease programs should enable the goals and objectives of this proposal to be achieved.